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The common, and with some exceptions, the only method to interconnect high power renewable energy sources and energy storage systems to the power system has been using power electronics converters that operate as current sources to the grid, for the purpose of achieving maximum primary-source power tracking. When grid is not available, such sources are not allowed to continue operating and are shut-down after anti-islanded algorithms recognize the loss of the grid. Although existing standards and requirements still limit grid-interface converters to regulate voltage in the grid, this functionality will most probably be the dominant mode of their operation in the future grid; hence the growing trend in industry and academia to concentrate research towards control of the grid-interface converters that resemble behavior of the classic synchronous generators. This paper addresses modeling of the virtual synchronous machine-based grid-interface converters for renewable energy systems integration. Being completely equivalent to the synchronous generator model, an average model of the grid-interface converter features an inherent frequency-locked loop, in which the virtual rotor angle is obtained by integrating dc-link voltage.
Cvetkovic et al. (Sun,) studied this question.